Abstract We present a compilation of spectroscopic observations of
the sgB[e] star
CI Cam
, the optical counterpart of
XTE J0421+560
. This includes data from before, during, and
after its 1998 outburst, with quantitative results spanning 37 years.
The object shows a rich emission line spectrum originating from
circumstellar material, rendering it difficult to determine the nature
of either star involved or the cause of the outburst. We collate all
available pre-outburst data to determine the state of the system
before this occurred and provide a baseline for comparison with
outburst and post-outburst data. During the outburst all lines become
stronger, and hydrogen and helium lines become significantly broader
and asymmetric. After the outburst, spectral changes persist for at
least three years, with Fe II and [N II] lines
still a factor of ~2 above the pre-outburst level and
He I, He II, and N II lines suppressed
by a factor of 2-10. We find that the spectral properties of
CI Cam
are similar to other sgB[e] stars and therefore
suggest that the geometry of the circumstellar material is similar to
that proposed for the other objects: a two component outflow, with a
fast, hot, rarefied polar wind indistinguishable from that of a normal
supergiant and a dense, cooler equatorial outflow with a much lower
velocity. Based on a comparison of the properties of
CI Cam
with the other sgB[e] stars we suggest that
CI Cam
is among
the hotter members of the class and is viewed nearly pole-on. The
nature of the compact object and the mechanism for the outburst remain
uncertain, although it is likely that the compact object is a black
hole or neutron star, and that the outburst was precipitated by its
passage through the equatorial material. We suggest that this
prompted a burst of supercritical accretion resulting in ejection of
much of the material, which was later seen as an expanding radio
remnant. The enhanced outburst emission most likely originated either
directly from this supercritical accretion, or from the interaction of
the expanding remnant with the equatorial material, or from a
combination of both mechanisms.